A typical wireless communication system includes one or more base stations, each radiating to define one or more coverage areas, such as cells and cell sectors, in which wireless client devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices (whether or not user operated), can operate. In turn, each base station could sit as a node on a core network that includes entities such as a network controller and a gateway system that provide packet-data-network connectivity. With this arrangement, a UE within coverage of the system could thus engage in air interface communication with a base station and thereby communicate via the base station with various remote network entities or with other UEs served by the system.
Such a network could operate in accordance with a particular radio access technology, with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link.
Over the years, the industry has embraced various generations of radio access technologies, in a continuous effort to increase available data rate and quality of service for end users. These generations have ranged from “1G,” which used simple analog frequency modulation to facilitate basic voice-call service, to “4G”—such as Long Term Evolution (LTE), which now facilitates mobile broadband service using technologies such as orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO). And most recently, the industry is now exploring developments in “5G” and particularly “5G NR” (5G New Radio), which may use a scalable OFDM air interface, advanced channel coding, massive MIMO, beamforming, and/or other features, to support higher data rates and countless applications, such as mission-critical services, enhanced mobile broadband, and massive Internet of Things (IoT).
In accordance with the radio access technology, each base station could be configured to operate on one or more radio-frequency (RF) carriers, each of which could be frequency division duplex (FDD), defining separate frequency channels for downlink and uplink communication, or time division duplex (TDD), with a single frequency channel multiplexed over time between downlink and uplink use. Each such frequency channel could have an associated center frequency and a bandwidth defining a width of frequency.
On the downlink and uplink, each such carrier could be structured to define various physical channels and air-interface resources for carrying information between the base stations and UEs. For instance, on the downlink, each carrier could define a reference channel on which the base station broadcasts a reference signal that UEs can evaluate to determine coverage strength, one or more downlink control channels for carrying control signaling from the base station to UEs, and one or more downlink traffic channels for carrying bearer communications from the base station to UEs. And on the uplink, each carrier could define one or more uplink control channels for carrying control signaling from UEs to the base station and one or more uplink traffic channels for carrying bearer communications from UEs to the base station.
When a UE initially powers on within such a system, the UE could scan various carriers in search of coverage, and upon detecting strong enough coverage of a base station, the UE could engage in signaling to establish a Radio Resource Control (RRC) connection with the base station and could then engage in an attach process if appropriate to register for service with the system and prepare the system to serve the UE.
In an example attach process, the UE could transmit an attach request over the air to the base station, and the base station could forward the attach request to the core-network controller for processing. And after authenticating and authorizing the UE, the network controller could then engage in signaling with the base station and with the gateway system, to coordinate establishment of a bearer (data tunnel) that that extends between the UE and the gateway system and defines an access point name (APN) connection associated with external transport-network connectivity. Further, a Dynamic Host Control Protocol (DHCP) server could assign to the UE an Internet Protocol (IP) address usable by the UE to engage in packet-data communication on the external transport network. And the base station and various core-network entities could each create and store a context record for the UE, identifying the UE's network connection, bearer identity, and other information to facilitate serving the UE.
Once a UE is so connected and attached, the base station could then provide the UE with wireless data communication service. For instance, when data arrives on the transport network destined to the UE's IP address, the gateway system could transmit the data via the UE's bearer to the base station, and the base station could then schedule downlink air interface resources to carry the data to the UE and transmit the data to the UE accordingly. Likewise, when the UE has data to transmit on the transport network, the UE could send a scheduling request to the base station, the base station could schedule uplink air interface resources to carry the data from the UE, and upon receipt of the data at the base station, the base station could then transmit the data via the UE's bearer to the gateway system for output onto the transport network.
These wireless communications between the UE and the base station could be carried out using a modulation and coding scheme (MCS) that is selected based on a quality of the air interface between the UE and the base station, with the MCS defining an effective data rate of the communications. For instance, if the UE is in relatively strong coverage of the base station, the base station could coordinate use of a relatively high-order MCS that will provide the UE with relatively high throughput. Whereas, if the UE is in relatively weak coverage of the base station, then the base station could coordinate use of a relatively low-order MCS that will provide the UE with relatively low throughput.